DE102015000852A1 - Freezing start method for a fuel cell system - Google Patents

Abstract

Disclosed are methods for ramping up a fuel cell system from below zero temperatures using the latent heat of crystallization available in a water supply maintained above the freezing temperature. During startup, a water spray subsystem is used to spray water from the reservoir onto a heat exchange surface in a heat exchange element through which coolant circulates out of a fuel cell stack coolant loop. The water freezes on the heat exchange surface, and the heat of crystallization is exchanged with the circulating coolant through the heat exchange surface, heating the coolant.

Description

background

Field of the invention

The present invention relates to methods of starting up a fuel cell system at sub-freezing temperatures. More particularly, it relates to methods for powering up an automotive fuel cell system that includes a solid polymer electrolyte fuel cell stack.

Description of the Related Art

Fuel cells such as solid polymer electrolyte or proton exchange membrane fuel cells electrochemically convert reactants, namely a fuel (such as hydrogen) and an oxidant (such as oxygen or air) to produce electrical power. Solid polymer electrolyte fuel cells generally use a proton conductive solid polymer membrane electrolyte between cathodic and anodic electrodes. A structure comprising a solid-polymer membrane electrolyte interposed between these two electrodes is referred to as a membrane-electrode assembly (MEA). In a typical fuel cell, flow field plates are provided on each side of the MEA, which include numerous fluid distribution channels for the reactants to distribute the fuel and oxidant to the respective electrodes, and to remove byproducts of the electrochemical reactions that take place within the fuel cell. Water is the major by-product in a fuel cell operated with hydrogen and air as reactants. Because the output voltage of a single cell is on the order of 1V, for commercial applications, usually a plurality of cells are stacked in rows to provide a higher output voltage. The fuel cell stacks may be further connected in groups of stacks connected together in series and / or parallel for use in automotive applications and the like.

Along with water, heat is a significant by-product of the electrochemical reactions that take place within the fuel cell. In general, therefore, means for cooling a fuel cell stack are needed. Stacks designed to achieve high power density (eg automobile stacks) typically circulate a liquid coolant through the stack to dissipate heat rapidly and efficiently. To accomplish this, typically, coolant flow fields including numerous coolant channels are also integrated into the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and so may distribute the coolant relatively evenly within the cells while reliably keeping the coolant separate from the reactants.

Various subsystems and methods have been disclosed in the art for the purposes of improving cooling performance in such stacks. For example, that describes CA2424172 a fuel cell system having a heat exchange unit coupled to the coolant inlet and outlet. A water spray unit is also provided to spray outlet water into air which is blown by the heat exchange unit. The outlet water is evaporated and increases the cooling capacity of the heat exchange unit.

In certain applications, PEMFC stacks may be subjected to repeated on-off duty cycles, which entail storage for various long periods of time and at different temperatures. It is generally desirable to be able to boot up such stacks within a short period of time. Certain applications, such as automotive applications, may require relatively quick and reliable startup from well below freezing storage conditions. This has presented a considerable challenge, both because of the comparatively low efficiency of the cells at such temperatures and because of the problems associated with the water balance in the cells when operating below 0 ° C. A certain amount of water is required for proper fuel cell operation (for example, hydration of the membrane electrolyte) and is generated as a result of providing electrical power. However, ice, of course, forms where liquid water is present at such temperatures. The presence of ice can be problematic when stored or when it is up depending on how much it is and where it is located.

Various designs of fuel cells and fuel cell power-up processes have been developed in the art to provide improved ramp up of below freezing temperatures. For example, the JP2005251463 A method of starting up a fuel cell system at a low temperature using a heat generating means, and wherein heat obtained from the solidification heat of sodium acetate trihydrate. Frozen water in the fuel cell stack is thawed with a low power consumption by starting the heat generating means before the stack is started up.

Despite the progress made so far, there continues to be a need for simpler and more effective methods of starting up fuel cell systems from a temperature below zero. The present invention provides a way to meet these needs and provides other related advantages.

Summary

As part of the process of starting up a fuel cell system from below zero temperatures, one can use the latent heat of crystallization present in a supply of water maintained above the freezing temperature. During startup, a water spray subsystem is used to spray water from the reservoir onto a heat exchange surface in a heat exchange element through which coolant circulates from a fuel cell stack coolant loop. The water freezes at the heat exchange surface, and the heat of crystallization is exchanged with the circulating coolant through the heat exchange surface, thereby heating the coolant and hence the fuel cell stack. Surprisingly, in an automotive fuel cell system, a modestly sized water supply can provide a substantial amount of the heat desired for start-up purposes.

In the present invention, the fuel cell system includes a fuel cell stack, a coolant circuit configured to circulate coolant through the fuel cell stack, and a heat exchange element in the coolant circuit in which the heat exchange element includes a heat exchange surface and coolant on one side of the heat exchange surface flows. The system further comprises a reservoir containing a supply of water and a water spray subsystem adapted to draw water from the water reservoir in the reservoir and to spray the water to the other side of the heat exchange surface. Specifically, then, the method of starting up such a fuel cell system from a temperature below freezing includes maintaining the supply of water above the freezing temperature before startup, circulating the refrigerant through the refrigerant circuit (and thus the fuel cell stack and the heat exchange element), drawing water therefrom Water supply in the container and the spraying of the water to the other side of the heat exchange surface, while the fuel cell system is at a temperature below freezing. As a result of the process, water freezes to the heat exchange surface, and the heat of crystallization is exchanged with the circulating coolant through the heat exchange surface, thereby heating the coolant.

During startup, an initial amount of power may be drawn from the fuel cell stack while the fuel cell system is at a temperature below freezing. Alternatively, the referencing of the power may be postponed until the system is above freezing to prevent the generation of water in the stack from the electrochemical reactions taking place therein.

In order to keep the water supply above the freezing temperature before start-up, the water reservoir is thermally well insulated (e.g. vacuum jacketed container). Furthermore, the fuel cell system may include an electric heater that is in thermal contact with the water supply. Heat from the electric heater can be used to hold the water supply above freezing.

In one embodiment, the water spray subsystem is also maintained above freezing temperature prior to start-up. This can also be achieved by using the above-mentioned electric heater (or other heater) when it is designed to be in sufficient thermal contact with the water spray subsystem.

In an alternative embodiment, the water spray subsystem may be allowed to fall to the same temperature below zero as the rest of the fuel cell system. Here, however, the water spray subsystem can be drained of water before the fuel cell system is exposed to a temperature below freezing. In this way, there is no water to be frozen in the Wassersprüh subsystem prior to startup.

The water spray subsystem may include a water pump and a spray nozzle. The water pump may desirably be self-priming, especially in embodiments in which the water spray subsystem is occasionally drained of water.

The process of the invention is generally suitable for use in systems containing solid polymer electrolyte Include fuel cell stack. For example, the method may be considered for use in an air cooled fuel cell system, which typically employs a solid polymer electrolyte fuel cell stack. In such an air-cooled mode, ambient air is typically obtained and used as both an oxidant and coolant. Here, when the present method is used, the heat exchange element may be the oxidant / coolant passages in the air-cooled fuel cell system. However, the method is particularly suitable for use in automotive fuel cell systems in which typically aqueous antifreeze liquid refrigerants are used.

In an automotive embodiment, a sufficient amount of latent heat may be expected to ramp up from a water reservoir that includes more than or about 0.03 liters of water per kW of power from the fuel cell stack. In certain preferred embodiments, the water supply may comprise less than or about 2 liters of water.

Often automotive fuel cell systems may already include elements in their coolant circuit which serve as suitable heat exchange elements for the present process. Such elements may require little to no modification to accommodate a suitable spray water subsystem. Alternatively, an element may be incorporated into the coolant loop for purposes of the present method.

Elements which often appear in coolant circuits and which serve as heat exchange elements may include: a contact humidifier, a charge air cooler and / or a radiator. Typically, a contact humidifier is disposed both in the coolant loop and in an oxidant inlet of the fuel cell stack. An intercooler may be employed in fuel cell systems that include an air compressor for providing compressed air at an oxidant inlet of the fuel cell stack. The charge air cooler is disposed between the air compressor and the oxidant inlet. A radiator is disposed at a suitable location in the coolant circuit to release heat to the environment.

Likewise, the fuel cell system may include elements that may serve, in part, as a water supply, container, and / or water spray subsystem for the present process. For example, as illustrated below, a fuel cell system may be employed that includes a U-shaped tube filled with water to provide a low pressure seal of an oxidant outlet of the fuel cell stack of the system. The water supply, the container and the spray water subsystem of the present invention can be elegantly integrated into such an arrangement without a large modification of the fuel cell system.

These and other aspects of the invention will be apparent by reference to the attached figure and the following detailed description.

Brief description of the drawings

1 FIG. 12 is a schematic illustration of an exemplary automotive fuel cell system that can be ramped from below freezing temperature using the method of the present invention.

Detailed description

In this description, words such as "a" and "comprising" are to be construed in an open sense, and are to be understood as meaning at least one but not limited to only one.

In the present case, in a quantitative context, the term "about" should be understood as lying in the range of up to plus 10% and up to minus 10%.

The process of the invention utilizes the latent heat of crystallization available in a supply of water which is maintained above a freezing temperature to aid in heating, and thus at startup, of a fuel cell system from below freezing temperatures. Water from the reservoir is sprayed onto a suitable heat exchange surface in a heat exchange element or elements through which coolant circulates out of a fuel cell stack coolant loop. The water freezes on the heat exchange surface, and the heat of crystallization is exchanged with the circulating coolant through the heat exchange surface, whereby the heating of the coolant takes place or is greatly assisted and the fuel cell system is started up.

An exemplary automotive fuel cell system that can be powered up using the method of the invention is shown schematically in FIG 1 shown. As shown, the fuel cell system includes 1 a fuel cell stack 2 which comprises a series stack of solid polymer electrolyte fuel cells. Ambient air is used as the oxidizer supply and is supplied by a compressor 10 compressed and an oxidant inlet 3a of the fuel cell stack 2 fed. This air can be significantly heated as a result of compression, and it is first cooled when so heated by passing through an intercooler 11 (at an inlet 11a ) and then it becomes a contact humidifier 12 (at an inlet 12a ), where it is moistened before finally passing the fuel cell stack 2 is supplied. Oxygen depleted air and by-product water vapor and liquid water are removed from the fuel cell stack 2 at an oxidant outlet 3b issued. This in 1 shown system sets an optional U-shaped tube 7 one, which with liquid water 8th filled as a simple low-pressure seal for the Oxidationsmittelauslass 3b serve the stack when the stack is shut down and not operated. About a valve 9 can water the U-shaped tube 7 supplied or drained from this. After flowing through the U-shaped tube 7 the oxidant offgas is used to drive the compressor 10 drive and it is then released into the environment. (The fuel supply, inlets and outlets as well as typical recirculation equipment in the fuel cell system have been omitted for the sake of simplicity 1 omitted.)

The cells in the fuel cell stack 2 include coolant flow fields (not shown) which suitably communicate with coolant manifolds (not shown) within the stack 2 are connected. During normal operation, circulating coolant is used to dissipate heat generated within the stack. When starting from a temperature below freezing, the circulating coolant can be used to charge the stack 2 to heat when the coolant is externally heated. The coolant used is typically an aqueous solution comprising a suitable antifreeze liquid (for example, ethylene glycol) and capable of withstanding the lowest expected ambient temperatures without freezing.

The coolant is added to the stack 2 at a coolant inlet 4a supplied, circulated within and then at a coolant outlet 4b dissipated. The coolant circulates outside the stack 2 through a coolant circuit 5 , A coolant pump 21 is used to convey the circulating coolant. During normal operation, the coolant that is inside the stack becomes 2 was heated to a cooler 14 led, where at a heat exchange surface 6c Heat is dissipated in the environment. Then, as in 1 shown, the coolant the intercooler 11 where it is used to cool the incoming air, when this occurs as a result of the compression in the compressor 10 was heated considerably. The surface where the incoming air is cooled, and where the heat exchange in the intercooler 11 takes place as a heat exchange surface 6b shown. The coolant is next to the contact humidifier 12 which is used to humidify the incoming air coming from the intercooler 11 comes. In the contact humidifier 12 Humidification is accomplished by spraying water or steam directly into the incoming air stream and / or onto a heated surface where it evaporates. As in 1 is shown in the coolant circuit 5 inside the contact humidifier 12 a surface 6a and serves as a heated surface for humidification purposes during normal operation and as a heat exchange surface 6a during startup in the present invention.

The fuel cell system 1 additionally includes a water spray subsystem 20 which is a thermally insulated container 15 (for example, a vacuum-jacketed container) comprising a supply of water 16 contains, which is kept above the freezing temperature. A spray line 17 is in the container 15 arranged to access the water supply 16 to have. A self-priming pump 22 is in the spray line 17 provided to remove water from the supply 16 to a spray nozzle 18 to pump, which within the contact humidifier 12 is arranged. As shown is the spray nozzle 18 designed to apply water to the heat exchange surface 6a for purposes of booting up the system 1 to spray according to the invention. However, spray water from the spray nozzle can 18 if this is suitably designed, as shown schematically in 1 also shown as moistening water for the contact humidifier 12 during normal operation or in addition to dampening water provided by other means. In addition, there is an electric heater 19 provided, which with the container 15 and the water supply 16 in thermal contact.

During normal operation, the fuel cell stack becomes 2 typically operated at temperatures well above ambient (for example 80 ° C). The coolant pump 21 pumps the antifreeze liquid so that it is in the coolant circuit 5 circulates and dissipates heat, which in the fuel cell stack 2 is produced. This heat is then transferred from the coolant to the environment via the radiator 14 issued. The coolant cooled by the radiator will then be in the intercooler 11 used to remove excess heat (if present as a result of the compression) from the incoming oxidizer air. The coolant, which the intercooler 11 leaves, then enters the contact humidifier 12 one and gets over the heat exchange surface 6a guided. Here, the coolant is used to heat the water which is on the heat exchange surface 6a is sprayed so it helps to vaporize the water and to humidify the incoming oxidant air. After leaving the contact humidifier 12 the coolant is returned to the fuel cell stack 2 guided.

Furthermore, the water supply is used during normal operation 16 as a water supply for humidification in the contact humidifier 12 , The liquid water 8th in the U-shaped tube 7 provides only moderate back pressure or restriction of the stream of oxidant offgas from the oxidant outlet 3b ,

If the fuel cell system 1 shut down, stored and / or ramped up at temperatures above freezing is the pump 22 typically off. However, the liquid water can 8th , the water reserve 16 and the rest of the spray water subsystem 20 be left as they are, and the electric heater 19 does not need to be used. In this situation, the liquid water 8th in the U-shaped tube 7 a low pressure seal for the oxidant outlet 3b ready and prevents entry of ambient air into the fuel cell stack 2 ,

However, if it is expected that the fuel cell system 1 Temperatures below zero experience when it is shut down, stored and / or powered up, steps are taken to freeze the water in the U-shaped tube 7 and the water spray subsystem 20 to prevent these events. For example, as in 1 shown the liquid water 8th over the valve 9 in the container 15 be drained and so freezing of water in the U-shaped tube 7 prevent. (Of course, the U-shaped tube is used 7 no longer to the oxidizer outlet 3b seal against the environment. If this is unacceptable, it may be necessary to use other means to remove the oxidizer outlet 3b seal). Furthermore, water may leak from the spray nozzle 18 , the self-priming pump 22 and the spray line 17 in the container 15 be drained. All this drained water and the rest of the water supply 16 be by means of the heat, which from the electric heater 19 is kept above the freezing temperature. Suitable equipment (not shown) for sensing and controlling the temperature may be used to ensure that the water supply 16 does not freeze without using excessive electrical energy.

In the in 1 In the embodiment shown, the coolant humidifier is used 12 as the heat exchange element for purposes of the invention. During startup from a temperature below zero, the coolant pump becomes 21 started, and the coolant is again through the coolant circuit 5 and so through the fuel cell stack 2 circulated. The coolant also flows on one side of the heat exchange surface 6a in the contact humidifier 12 , The self-priming pump 22 is also started, and water above the freezing temperature becomes out of the liquid water supply 16 pumped and out of the spray nozzle 18 on the heat exchange surface 6a sprayed. The sprayed water freezes on the heat exchange surface 6a and the heat of crystallization is exchanged with the circulating coolant while heating it. The heated coolant then becomes the coolant inlet 4a where it is now the fuel cell stack 2 heated.

The process then continues until the coolant and fuel cell stack 2 have reached almost zero degrees, followed by, for example, heat from the operation of the fuel cell stack 2 bring the rest of the system above freezing and to normal operating temperature. Thus, an initial amount of power from the fuel cell stack 2 while the system is just below freezing.

Even though 1 and the foregoing description illustrates one possible embodiment of the invention, those skilled in the art will recognize that other system arrangements and / or other startup procedures may be considered. For example, the heat exchange element used during startup may be the intercooler 11 , the cooler 14 or an additional particular element in the system or additionally. In such a case would be the water spray subsystem 20 designed to water on the surfaces 6b . 6c and / or to spray onto a suitable surface in the additional particular element (not shown). Furthermore, the optional U-shaped tube needs 7 not to be used. In addition, the electric heater 19 or an additional electrical heater to be designed, including the rest of the water spray subsystem 20 to heat, so that the water does not need to be drained from this, and also the use of a pump 22 which is not self-priming. And even moreover, additional heat may be obtained for purposes of power-up by other means, which is the Are known in the art. For example, power does not need to come from the fuel cell stack 2 until it has reached a temperature above freezing.

Perhaps surprisingly, calculations indicate that a modest amount of water can be expected to provide a sufficient amount of latent heat to power up a typical automotive fuel cell stack. For example, a supply of water containing more than or about 0.03 liters per kW of the capacity of the fuel cell stack and maintained above the freezing temperature may have a sufficient amount of latent heat. In certain preferred embodiments, therefore, the water supply may comprise less than or about 2 liters of water.

All of the above US patents, publications of US patent applications, US patent applications, foreign patents, foreign patent applications, and non-patent literature referenced in this specification are hereby incorporated by reference in their entirety.

Although particular elements, embodiments and applications of the present invention have been shown and described, it is to be understood that the invention is not limited thereto, as modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the art preceding teachings. For example, although the above description has been directed primarily to liquid cooled fuel cell systems, it is possible to contemplate the disclosed methods for air cooled or other fuel cell systems as well. Such modifications are to be considered within the scope and scope of the following claims.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• CA 2424172 [0004]
• JP 2005251463 [0006]

Claims (17)

A method of powering up a fuel cell system from a temperature below freezing, the fuel cell system comprising: a fuel cell stack, a coolant circuit configured to circulate a coolant through the fuel cell stack, a heat exchange element in the coolant loop, the heat exchange element comprising a heat exchange surface and coolant flows on one side of the heat exchange surface, a reservoir containing a water supply, a water spray subsystem adapted to draw water from the water reservoir in the reservoir, and to supply the water to the other side of the heat exchange surface spraying, the method comprising: Keeping the water supply above the freezing temperature before start-up; Circulating the coolant through the coolant circuit, the fuel cell stack, and the heat exchange element; Obtaining water from the water supply in the container; and Spraying the water to the other side of the heat exchange surface while the fuel cell system is at a temperature below freezing. The method of claim 1, wherein the water freezes on the heat exchange surface and the heat of crystallization is exchanged with the circulating coolant through the heat exchange surface, wherein the coolant is heated. The method of claim 1 comprising: Obtaining an initial amount of power from the fuel cell stack while the fuel cell system is at a temperature below freezing. The method of claim 1, wherein the fuel cell system comprises an electric heater that is in thermal contact with the water supply, and wherein the method comprises maintaining the water supply above the freezing temperature by using heat from the electric heater prior to startup. The method of claim 1 comprising: Keep the water spray subsystem above the freezing temperature before startup. The method of claim 1 comprising: Discharge water from the water spray subsystem before exposing the fuel cell system to a temperature below freezing. The method of claim 1, wherein the water spray subsystem comprises a water pump and a spray nozzle. The method of claim 7, wherein the water pump is self-priming. The method of claim 1, wherein the water supply comprises more than or about 0.03 liters of water per kW of power from the fuel cell stack. The method of claim 1, wherein the water supply comprises less than or about 2 liters of water. The method of the fuel cell system of claim 1, wherein the fuel cell stack is a solid polymer electrolyte fuel cell stack. The method of claim 11, wherein the fuel cell system is an automotive fuel cell system. The method of claim 12, wherein the heat exchange element is a contact humidifier, which is arranged both in the coolant circuit and in an oxidant inlet of the fuel cell stack. The method of claim 12, wherein the fuel cell system comprises an air compressor for providing compressed air at an oxidant inlet of the fuel cell stack, and wherein the heat exchange element is a charge air cooler disposed between the air compressor and the oxidant inlet. The method of claim 12, wherein the heat exchange element is a radiator, which is arranged in the coolant circuit of the fuel cell stack. The method of claim 1, wherein the coolant is an antifreeze liquid. The method of claim 1, wherein the container is thermally insulated.

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